Nuclear dimorphism occurs in the ciliated protozoa. A major problem concerns the relative roles of the micronucleus and macronucleus during the life-cycle as well as in the evolution of these organisms. The macronucleus, the functional somatic nucleus, is periodically discarded and replaced by products of the micronucleus, the germ-line nucleus, during normal conjugation. Part of the program is directed toward a biochemical understanding of the basis for nuclear differentiation. Specificially, our concern is with the organization and kinds of DNA molecules within each nucleus and within differentiated macronuclei, principally in the species-complex Tetrahymena pyriformis, but also in Paramecium aurelia. Our efforts will be directed at (a) comparing the DNA's of micronuclei and macronuclei in terms of physical properties, genomic content and nucleotide sequence specificity, (b) comparing macronuclear DNA's from phenotypically differentiated cells within a clone as to nucleotide sequence specificity, and (c) comparing macronuclear DNA's from cells of different age within a clone. A second part of the program is concerned with comparing changes in isozymes and DNA sequences within species-complexes, particularly Paramecium aurelia, in order to measure evolutionary distances between sibling species. The possible effect of nuclear differentiation on molecular evolution will be explored by comparisons of nucleotide sequences in purified nuclei.